This invention relates generally to refrigeration systems and in particular to a refrigeration apparatus having an electrically controlled expansion valve disposed to selectively control refrigerant flow through the system to optimize refrigerant level in an evaporator so as to enhance energy efficiency of the system.
Conventional refrigeration systems having moderate capacity (e.g., less than 5 tons) and typically include a compressor, a condenser, an evaporator, and a fixed expansion device, such as an orifice or capillary tube. The fixed expansion device is positioned so as to introduce a pressure drop in the refrigerant as it passes from the condenser to the evaporator. One factor in determining energy-efficient operation of the refrigeration system is determined by the phase change of the refrigerant from liquid to vapor in the evaporator. On one hand, it is undesirable that liquid refrigerant pass from the evaporator as that would indicate that the cooled air did not reject all of the energy that it could have to the refrigerant (also, liquid refrigerant would damage the compressor); on the other hand, it is undesirable that the refrigerant be significantly superheated as the superheated refrigerant has less density and thus more work is required to compress it (because less mass of refrigerant would be pulled into the cylinder on each intake stroke of the compressor). The optimal evaporator efficiency is obtained when all liquid refrigerant passing into the evaporator undergoes a phase change to vapor, thereby absorbing heat from the cooling air circulating around the other side of the evaporator heat exchanger, but the refrigerant vapor has the least superheat consistent with other evaporator operating constraints (e.g., not passing any liquid to the compressor).
In refrigeration systems used in refrigerators and small heat pump systems (which typically have a capacity of one-half ton or less), the compressor speed is also fixed and thus such systems can be tuned for most energy-efficient operation for only one temperature differential across the evaporator, and that temperature differential is necessarily based on a worse case scenario to avoid product malfunctions under heavy load conditions. As a consequence, under more common normal operating conditions, the refrigeration system does not operate at peak energy efficiency.
Conventional refrigeration systems also typically do not reach their top efficiency until many minutes into an operating cycle (that is, 5-10 minutes after the system has been actuated to address a cooling demand). This delay in reaching an efficient operating point results from the accumulation of vapor in the evaporator after system startup (when the compressor suction causes what liquid refrigerant that is left in the evaporator at the previous shutdown to flash to vapor) and the resultant time to pump the vapor out of the evaporator and raise the level of liquid refrigerant in the evaporator.
It is desirable to improve the energy-efficiency of refrigeration systems by enabling them to meet a range of cooling demands and environmental conditions by controlling the system to respond to the current cooling demands. It is also desirable that the refrigeration system be able to reach an efficient operating regime early in a cooling cycle. The energy saving system should also be readily fabricated and easily adapted to the refrigeration systems presently manufactured such that the cost of acquiring and operating the system does not exceed the economic benefits of the improved energy efficiency.
It is thus an object of this invention to provide a refrigeration system that improves the energy efficiency of the system through selectively controlling refrigerant flow with a variable expansion device.